Ive finished all the levels in the single player campaign. Im not about to fork over 2-4 bucks for each additional map pack though, or $3 for keys to open dropboxes.
Its a shame, because I really do enjoy the game, and would totally pay $5 for an unlocked version.
You could also improve it by being able to sell duplicate mercs/weapons, and adding more weapons in general. Also, when deploying a gun or rocket turret you should be able to check its line of fire, to make sure you actually want to place it there.